The Ice Age and Its Work.
I. Erratic Blocks and Ice-Sheets. (S481: 1893)

Editor Charles H. Smith's Note: The first portion of a long subject review that appeared in two
parts in the 1 November and 1 December 1893 issues of Fortnightly Review. Original pagination
indicated within double brackets. To link directly to this page connect with:
http://people.wku.edu/charles.smith/wallace/S481A.htm

[[p. 616]] It is little more than fifty years ago that one of the
most potent agents in modifying the surface features of our country was
first recognised. Before 1840, when Agassiz accompanied Buckland to Scotland,
the Lake District, and Wales, discovering everywhere the same indications
of the former presence of glaciers as are to be found so abundantly in
Switzerland, no geologist had conceived the possibility of a recent glacial
epoch in the temperate portion of the northern hemisphere. From that year,
however, a new science came into existence, and it was recognised that
only by a careful study of existing glaciers, of the nature of the work
they now do, and of the indications of the work they have done in past
ages, could we explain many curious phenomena that had hitherto been vaguely
regarded as indications of diluvial agency. One of the first fruits of
the new science was the conversion of the author of Reliquiæ
Diluvianæ--Dr. Buckland, who, having studied the work of glaciers
in Switzerland in company with Agassiz, became convinced that numerous
phenomena he had observed in this country could only be due to the very
same causes. In November, 1840, he read a paper before the Geological
Society on the "Evidences of Glaciers in Scotland and the North of England,"
and from that time to the present the study of glaciers and of their work
has been systematically pursued with a large amount of success. One after
another crude theories have been abandoned, facts have steadily accumulated,
and their logical though cautious interpretation has led to a considerable
body of well-supported inductions on which the new science is becoming
firmly established. Some of the most important and far-reaching of these
inductions are, however, still denied by writers who have a wide acquaintance
with modern glaciers; and as several works have recently appeared on both
sides of the controversy, the time seems appropriate for a popular sketch
of the progress of the glacial theory, together with a more detailed discussion
of some of the most disputed points as to which it seems to the present
writer that sound reasoning is even more required than the further accumulation
of facts.1

[[p. 617]] In the last century, Swedenborg, Linnæus, Pallas,
De Luc, and many other eminent writers took notice of the remarkable fact
that in Scandinavia, Russia, Germany, and Switzerland detached rocks or
boulders were found, often in great abundance and of immense size, and
of a kind that did not exist in situ in the same district, but
which were often only to be discovered in remote localities, sometimes
hundreds of miles away. Those who ventured to speculate on the origin
of these travelled rocks usually had recourse to water-power to account
for their removal; and as their large size and often elevated position
required some unusual force to carry them, there arose the idea of enormous
floods sweeping over whole continents; and for a long time this diluvial
theory was the only one that appeared to be available, although the difficulties
of its application to explain all the phenomena became greater the more
closely those phenomena were studied. Still, there was apparently no other
known or conceivable means of accounting for them, and for the enormous
mounds of gravel or clay intermixed with boulders which often accompanied
them; and the efforts of geologists were therefore directed to the discovery
of how the water-power had acted, and by what means the supposed floods
could have been produced.

There were not wanting
men who saw that no action of water alone could account for the facts.
Sir James Hall pointed this out with regard to erratics on the Jura, whose
source was undoubtedly in the far-distant Alps; and Mr. Grainger, in America,
described some of the parallel grooves and flutings running for nearly
a mile in Ohio, strongly arguing that no action of running water could
have produced them, but that an agent was required the direction of whose
movement was fixed and unalterable for long distances and for a great
length of time. No light was, however, thrown on the problem till 1822,
when Venetz, a Swiss engineer, finding that existing glaciers varied in
extent from year to year and that historical records showed them to have
considerably increased during the last eight centuries, was further led
to observe that, long before the historical era the glaciers had been
immensely more extensive, as shown by the smooth and rounded rocks, by
longitudinal scratches and grooves pointing down the valleys, and by numbers
of old moraines exactly similar in form and materials to those deposited
by existing glaciers. He read a paper before the Helvetic Society of Natural
History, and urged that glaciers once stretched down the Rhone valley
as far as the Jura, and there deposited the erratic blocks which had so
puzzled the diluvialists to explain.

Other writers soon followed
the clue thus given. In 1835 Charpentier, after a close study of the erratic
blocks and of their sources, adopted the views of Venetz. Agassiz followed,
and by his strenuous advocacy did much to spread correct views as to the
former [[p. 618]] extension of the Alpine
glaciers, and their capability of explaining the numerous superficial
phenomena which in all northern countries had been thought to afford proofs
of enormous floods and of the submergence of a large part of Europe under
a deep sea. He has, therefore, gained the reputation of being the originator
of the modern school of glacialists, which undoubtedly owes much to his
energy, research, and powers of exposition, though all the more important
facts, as well as the logical conclusions to be drawn from them, had been
pointed out by previous writers.

Before proceeding further,
it will be well to give a brief outline of the phenomena which lead to
the conclusion that glaciers have formerly existed in districts and countries
where even perpetual snow on the mountain tops is now unknown. These may
be briefly classed as--(1) Moraines and drifts; (2) Rounded, smoothed,
or planed rocks; (3) Striæ, grooves, and furrows on rock-surfaces;
(4) Erratics and perched blocks.

(1) Moraines are those heaps or ridges of rock and other debris which are deposited on the
surface of a glacier from the precipices or mountain slopes which border it, and which form what
are termed lateral and medial moraines while upon it, and terminal moraines when, being
gradually discharged at its end, either from above or from beneath it, they form great heaps of
rock and gravel corresponding in outline and extent to that of the terminal ice-cliff. Such
moraines can be seen on and near all existing glaciers, and their mode of formation and
characteristics are perfectly well known. If the glacier is continuously retreating, then the
terminal moraine will form more or less irregular heaps over the surface the glacier has formerly
covered; but when, as is usually the case, the glacier remains stationary for a considerable period,
then the terminal moraine will have a definite form, and will often stretch quite across the valley,
but presenting one or more openings through which the glacier stream has cut its way. Such
moraines form steep mounds, usually curved and often very regular, seeming from a little
distance to block up the valley like an artificial earthwork. Among hundreds that might be
enumerated good examples may be seen in Glen Isla (Forfarshire), in the Troutbeck valley near
Windermere, and in Cwm Glas, on the north side of Snowdon, this latter being so regularly
curved, evenly sloped, and level-topped as to look from below exactly like an ancient
fortification. The characteristic features of moraines are their position in valleys where there are
other indications of glacial action, their steep slopes and often level tops, but especially their
composition of earth, stones, and gravel, with large fragments of rock irregularly scattered
through them from top to bottom without any sign of stratification, while usually one or more
large blocks rest upon their summits in [[p. 619]] positions where they could only have been left
by the retreat of the glacier, or possibly stranded from floating ice. Where extensive glaciers have
covered large areas of nearly level ground the moraines form great sheets extending for many
miles, often concealing the original contours of the country, and then receive the general name of
drift. The composition of drift is usually the same as that of well-marked moraines, large blocks
of stone being distributed throughout its mass. It is this which mainly distinguishes drift from
alluvial or shore deposits, in which the materials are always more or less assorted and stratified;
but the angular forms of many of the contained blocks and the striated surfaces of others are also
characteristic. Besides the terminal moraines of extinct glaciers, lateral moraines are also left
along the slopes of open valleys from which glaciers have retreated. As a whole, moraines are
well distinguished from all accumulations formed by water, and it has not been shown that any
other agency than glaciers is capable of forming them. In all recently glaciated countries they are
to be found more or less frequently, and thus afford an excellent first indication of the former
existence of glaciers.

(2) Smoothed and rounded
rocks, called in Switzerland "roches moutonnées," from their supposed
resemblance at a distance to sheep lying down, are perhaps the most general
of all the indications of glacial action. Every glacier carries with it,
imbedded in its under surface, numbers of rocks and stones, which, during
the slow but unceasing motion over its bed, crush and grind down all rocky
projections, producing in the end gently rounded or almost flat surfaces
even on the hardest and toughest rocks. In many of the valleys of Wales,
the Lake District, and Scotland every exposed rock has acquired this characteristic
outline, and the same feature can be traced on all the rocky slopes, and
often on the summits of the lesser heights; and the explanation of how
these forms have been produced is not a theory only, but has been observed
in actual operation in the accessible portions of many glaciers. Rocks
and stones are to be seen embedded in the ice and actually scratching,
grooving, and grinding the rock beneath in their slow but irresistible
onward motion. The rocky islets in Windermere, Ullswater, and other lakes,
as well as the Thousand Islands of the St. Lawrence, are thus ice-ground;
and the amount of the grinding can often be seen to be proportional to
the pressure and motion of the advancing glacier. I recently noticed in
the marshy alluvial plain above Derwentwater a projecting rock which has
been ground down to so regular a curve as to look like a portion of an
enormous globe buried in the earth. By rough measurement and estimate
this rock was about 250 feet across, and 20 or 30 feet high. It was formed
of hard slate, with numerous quartzite veins, the whole ground down to
a uniform [[p. 620]] spherical surface. It
had evidently once been an island in the lake, having a much broader base
now hidden by the alluvium, and may originally have been one of those
abrupt craggy rocks a few hundred feet high, which, owing to their superior
hardness or tenacity, resisted ordinary denudation, and which, when above
the old ice-level, form those numerous "pikes" which add so much to the
wild and picturesque scenery of the district. Looking at such rocks as
this, with outlines so utterly unlike any that are produced in similar
formations by sub-aerial denudation--and they are to be seen by scores
in all glaciated regions--we cannot but conclude that the ice-tool has
done more than merely rub off the angles and minor prominences, and that
it has really ground away rocky hills to an unknown but very considerable
extent; and this conclusion is, as we shall see, supported by a very large
amount of confirmatory evidence. It may be noted that ice-ground rocks
usually show the direction in which the ice has moved, by the side opposed
to the motion being more completely smoothed than the lee side, which
often retains some of its ruggedness, having been protected partly by
the ice overriding it and partly by the accumulation of its own debris.
Where such rocks occur in the higher parts of valleys the smooth side
always looks up the valley from which the glacier has descended. In the
more open parts of valleys, or in high coombs or cirques, where two or
more small ravines meet and where the ice may have been embayed and have
acquired a somewhat rotary motion, the rocks are seen to be ground down
on all sides into smooth mammillated mounds or hummocks, showing that
the ice has been forced into all the irregularities of the surface. An
example on a small scale is to be seen in Cwm Glas, on the north side
of Snowdon, above the fine moraine already mentioned, and in many other
places around the same mountain. On the whole, considering their abundance
in all glaciated regions, and the amount of information they give as to
the direction and grinding power of ice, these rounded rocks afford one
of the most instructive indications of the former presence of glaciers;
and we must also agree with the conclusion of Darwin (in a paper written
after studying the phenomena of ice-action in North Wales, and while fresh
from his observations of glaciers and icebergs in the southern hemisphere)
that "one of the best criterions between the effects produced by the passage
of glaciers and of icebergs is boss or dome-shaped rocks."

(3) Striated, grooved, and fluted rocks, though closely connected with the preceding, form a
distinct kind of evidence of the greatest value. Most of the bosses of rock just described have
been exposed to the action of the atmosphere, perhaps since the ice left them, and have thus
become more or less roughened or even disintegrated; but where the rocks have been protected
by a covering of drift, or even [[p. 621]] of turf, and have been recently exposed, they often
exhibit numerous parallel striæ, varying from the finest scratches to deep furrows a foot or more
in diameter. Fine examples are to be seen near the lakes of Llanberis, and they occur more or less
frequently in every glaciated country. Perhaps none of the effects of ice so clearly demonstrate
the action of glaciers as opposed to that of icebergs, owing to the general constancy of the
direction of the striæ, and the long distances they may be traced up and down slopes, with a
steadiness of motion and evenness of cutting power which no floating mass could possibly exert.
Sir A. Geikie tells us that in Gareloch, Bute, and Cantyre the striations on the rocks run up and
over the ridges, and are as clearly shown on the hill-tops as in the valleys. Mr. D. Mackintosh
states (in his paper on the "Ice-sheet of the Lake District and of North Wales") that in the valley
above Windermere the striæ cross Rydal Fell, Loughrigg Fell, and Orrest Head, ascending and
descending their slopes, often obliquely. But it is in the United States that the most remarkable
rock-groovings are to be found, extending over a large portion of the North-eastern States. In his
report on "The Rock-scorings of the Great Ice Invasions" Mr. T. C. Chamberlin gives many fine
illustrations, from photographs, showing striæ and grooves along sloping, curved, or vertical
surfaces, the striæ following the changes of curve, so that the grinding material must have been
slowly forced into close contact with the irregular surface. Of one of these examples Mr.
Chamberlin says:--

"The
climax of adaptability is reached in the striation of warped and twisted
surfaces, and of tortuous valleys. One of the most remarkable known instances
of this within the limits of photographic illustration is furnished by
the great glacial grooves at Kelly's Island (Fig. 17). These exhibit not
only the pliancy of the ice, but at the same time its strong hold upon
the armature with which it did its work of abrasion, grooving, and striation.
For, while these grooves can scarcely be supposed to have been originated
de novo by the gouging action of the ice, they are, nevertheless,
ploughed with deep furrows, the symmetry, continuity, and peculiar form
of some of which are only intelligible on the supposition that they were
cut by a single graving tool, held with sufficient tenacity by the ice
to execute by a single movement a deep, sharply-defined groove. There
is, perhaps, no finer illustration of the pliancy with which the ice yielded
to its encompassing barriers, the tenacity with which it held its armature,
and withal the pressure that both forced it into compliance with its tortuous
channel, and pressed it relentlessly forward."2

Kelly's Island is
at the western end of Lake Erie, and in the direction of the striæ
to the north-east there is no high ground for about 400 miles. Looking
at these facts, I cannot give any weight to the opinions of these who,
from observations of existing glaciers, [[p. 622]]
declare positively that ice cannot go up-hill, and can exert
no grinding power on level ground.

(4) Erratic blocks were
among the phenomena that first attracted the attention of men of science.
Large masses of granite and hard metamorphic rock, which can be traced
to Scandinavia, are found scattered over the plains of Denmark, Prussia,
and Northern Germany, where they rest either on drift or on quite different
formations of the Secondary or Tertiary periods. One of these blocks,
estimated at 1,500 tons weight, lay in a marshy plain near St. Petersburg,
and a portion of it was used for the pedestal of the statue of Peter the
Great. In parts of North Germany they are so abundant as to hide the surface
of the ground, being piled up in irregular masses forming hills of granite
boulders, which are often covered with forests of pine, birch, and juniper.
Far south, at Fürstenwalde south-east of Berlin, there was a huge
block of Swedish red granite, from one half of which the gigantic basin
was wrought which stands before the New Museum in that city. In Holstein
there is a block of granite 20 feet in diameter; and it was noticed by
De Luc that the largest blocks were often found at the greatest distance
from the parent rock, and that this fact was conclusive against their
having been brought to their present position by the action of floods.

It is, however, in Switzerland
that we find erratic blocks which furnish us with the most conclusive
testimony to the former enormous extension of glaciers: and as these have
been examined with the greatest care, and the facts, as well as the main
inductions from the facts, are generally admitted by all modern writers,
it will be well to consider them somewhat in detail. It will be found
that they give us most valuable information both as to the depth and extension
of ancient glaciers, and also as to the possibilities of motion in extensive
ice-sheets.

The most important of
these facts relate to the erratic blocks from the higher Alps, which are
found on the flanks of the Jura Mountains wholly formed of limestone,
on which it is therefore easy to recognise the granites, slates, and old
metamorphic rocks of the Alpine chain. These erratic blocks extend along
the Jura range for a distance of 100 miles, and up to a height of 2,015
feet above the Lake of Neufchatel. The first important point to notice
is that this highest elevation is attained at a spot exactly opposite,
and in the same direction as, the Rhone valley, between Martigny and the
head of the Lake of Geneva, while north or south of this point they gradually
decline in elevation to about 500 feet above the lake. The blocks at the
highest elevation and central point can be traced to the eastern shoulder
of Mont Blanc. All those to the south-west come from the left-hand side
of the lower Rhone valley, while those [[p. 623]]
to the north-east are all from the left side of the upper Rhone valley
and its tributaries. Other rocks coming from the right-hand side of the
upper Rhone valley are found on the right-hand or Bernese side of the
great valley between the Jura and the Bernese Alps.3

Now, this peculiar and definite distribution, which has been worked out with the greatest care by
numerous Swiss geologists, is a necessary consequence of well-known laws of glacier motion. The
debris from the two sides of the main valley form lateral moraines which, however much the glacier
may afterwards be contracted or spread out, keep their relative position unchanged. Each important
tributary glacier brings in other lateral moraines, and thus when the combined glacier ultimately
spreads out in a great lowland valley the several moraines will also spread out, while keeping their
relative position, and never crossing over to mingle with each other. So soon as this definite position
of the erratics was worked out it became evident that the first explanation--of a great submergence
during which the lower Swiss valleys were arms of the sea and the Rhone glacier broke off in
icebergs which carried the erratics across to the Jura--was altogether untenable, and that the original
explanation of Venetz and Charpentier was the true one. Sir Charles Lyell, who had first adopted the
iceberg theory, gave it up on examining the country in 1857 and ascertaining that the facts were
correctly stated by the Swiss geologists; and there is at the present day no writer of the least
importance who denies this. Sir Henry Howorth, who is one of the strongest opponents of what he
considers the extreme views of modern glacialists, gives a full summary of the facts as to the old
Rhone glacier from Charpentier. He states that between Martigny and St. Maurice the moraine debris
on each side of the valley shows the glacier to have reached a height of 3,000 feet above the river;
farther on, where the valley widens over the Lake of Geneva, it sank to 2,600 feet, while on the Jura
itself it seems to have been again raised to 3,000 feet at its highest point;4 and he quotes
Charpentier's general conclusion:--

"It
goes without saying that not only all the valleys of the Valais were filled
with ice up to a certain height, but that all lower Switzerland in which
we find the erratic debris of the Rhone valley must have been covered
by the same glacier. Consequently all the country between the Alps and
the Jura, and between the environs of Geneva and those of Soleure, has
been the bed of a glacier."

And then, after quoting the observations of Agassiz on the same phenomena and of those of North
America, he gives his own conclusions in the following words:--

[[p.
624]] "It is plain to those who would look without prejudice that
the rounded and mammillated surfaces, the scratched, polished, and grooved
rocks, and a great number of the phenomena which accompanied the distribution
of the boulders and the drift, are consistent only with the fact that
in the last geological age there was an immense development of glaciers
which occupied not only the high ranges of the Alps and the Dovrefelds,
but the secondary ranges and lower heights of the continents of Europe
and North America. This conclusion seems supported by every form of converging
evidence, and is apparently beyond the reach of cavil. So far there is
no question at issue."5

We may take it, therefore, that the views of Charpentier, Agassiz, and Sir Charles Lyell as to the
extent and thickness of the great Rhone glacier are admitted to be correct, or, at least, not to be
exaggerated, by the most strenuous opponents of the extreme glacialists. We may, therefore, use this
as a fixed datum in our further investigations, and I think it will be found to lead us irresistibly to
conclusions which in other cases these writers declare to be inadmissible.

We must now consider
briefly the distribution of erratics in North America, because they present
some peculiar features and teach us much concerning the possibilities
of glacier motion.

An immense area of the
North-eastern States, extending south to New York, and then westward in
an irregular line to Cincinnati and St. Louis, is almost wholly covered
with a deposit of drift material, in which rocks of various sizes are
embedded, while other rocks, often of enormous size, lie upon the surface.
These blocks have been carefully studied by the American geologists, and
they present us with some very interesting facts. Not only are the distances
from which they have been transported very great, but in very many cases
they are found at a greater elevation than the place from which they must
have come. Professor G. F. Wright found an enormous accumulation of boulders
on a sandstone plateau in Monroe County, Pennsylvania. Many of these boulders
were granite, and must have come either from the Adirondack Mountains
200 miles to the north, or from the Canadian Highlands still farther away.
This accumulation of boulders was 70 or 80 feet high, and it extended
many miles, descending into a deep valley 1,000 feet below the plateau
in a nearly continuous line forming part of the southern moraine of the
great American ice-sheet.

On the Kentucky hills,
about twelve miles south of Cincinnati, conglomerate boulders containing
pebbles of red jasper can be traced to a limited outcrop of the same rock
in Canada to the north of Lake Huron, more than 600 miles distant, and
similar boulders have been found at intervals over the whole intervening
country. In both these cases the blocks must have passed over intervening
valleys and hills, the latter as high or nearly as high as the source
[[p. 625]] from whence the rocks were derived.
Even more remarkable are numerous boulders of Helderberg limestone on
the summit of the Blue Ridge in Pennsylvania, which must have been brought
from ledges at least 500 feet lower than the places upon which they now
lie. The Blue Ridge itself shows remarkable signs of glacial abrasion,
in a well-defined shoulder marking the southern limit of the ice (as indicated
also by heaps of drift and erratics), so that Mr. Wright concludes that
several hundred feet of the ridge have been worn away by the ice.

The crowning example
of boulder transportation is, however, afforded by the blocks of light
grey gneiss discovered by Professor Hitchcock on the summit of Mount Washington,
over 6,000 feet above sea-level, and identified with Bethlehem gneiss,
whose nearest outcrop is in Jefferson, several miles to the north-west,
and 3,000 or 4,000 feet lower than Mount Washington.

These varied phenomena
of erratic blocks and rock striations, together with the enormous quantity
of boulder-clay and glacial drift spread over the whole of the Eastern
States, terminating southward in a more or less abrupt line of mounds
having all the characteristics of an enormous moraine, have led American
geologists to certain definite conclusions in which they all practically
agree. It may be well first to give a notion of the enormous amount of
the glacial debris under which a large part of the Eastern States is buried.
In New England these deposits are of less thickness than farther south,
averaging from 10 to 20 feet over the whole area. In Pennsylvania and
New York east of the Alleghanies, the deposits are very irregular, often
60 or 70 feet thick and sometimes more. West of the Alleghanies, in New
York, Pennsylvania and Ohio the thickness is much greater, being often
150 or 200 feet in the wide valleys, and 40 or 50 feet on many of the
uplands. Professor Newberry calculates that in Ohio it averages 60 feet
deep over an area of 25,000 square miles.

The direction of the
striæ and of the travelled boulders together with the form of the
great terminal moraines show that there must have been two main centres
of outflow for the ice-sheet, one over Labrador, the other over the Laurentian
Highlands north of Lake Superior. The southern margin of the drift may
be roughly represented by portions of circles drawn from these two points
as centres. The erratics on the summit of Mount Washington show that the
ice-sheet must have been a mile thick in its neighbourhood, and much thicker
at the centres of dispersion, while the masses of drift and erratics on
plateaus 2,000 feet high near its southern boundary indicate a great thickness
at the termination. The Laurentian plateau is now about 2,000 feet above
the sea-level, but there are numerous indications from buried river channels,
filled with drift and [[p. 626]] far below
the sea, which lead to the conclusion that during the Ice Age the land
was much higher. That snow can accumulate to an enormous extent over land
of moderate height when the conditions are favourable for such an accumulation
is shown by the case of Greenland, the greater part of whose surface is
a vast plateau of ice flowing outward by numerous glaciers into the sea.
The centre of this plateau where Dr. Nansen crossed it was over 9,000
feet above sea-level, and it may be very much higher farther north. It,
therefore, seems probable that the great American ice-sheet was, at least,
as high, and perhaps much higher, and this would give sufficient slope
for the flow to the southern border. Of course, during the successive
stages of the glaciation there may have been numerous local centres from
which glaciers radiated, and during the passing away of the Ice Age these
local glaciers would have left striæ and other indications of their
presence. But so much of the area covered by the drift--all, in fact,
south of the New England mountains and the Great Lakes--is undulating
ground, hill, valley, and plateau of moderate height that here all the
phenomena seem to be due to the great confluent ice-sheet during the various
phases of its advance and its passing away.

Sir Henry Howorth, in
his very instructive work already quoted, denies the existence and even
the possibility of such ice-sheets as those here indicated as having occurred
in North America and Europe. He maintains that ice of the requisite thickness
could not exist, as it would be crushed or liquefied by its own weight;
and further, that if it existed it could not possibly move over hundreds
of miles of generally level country, passing over hills and valleys and
carrying with it, either on its surface or in its lower strata, the enormous
quantity of boulders, gravel, and clay which we find everywhere overlying
the present surface of the ground. No doubt the difficulty does seem an
enormous one, but I think that it can be shown to be not so great as it
seems; and it is certainly by no means so insuperable as that of the apocryphal
floods, or "waves of translation" as they have been called, to which he
imputes the phenomena. He asks us to believe in one or more gigantic waves
sweeping over Eastern North America, carrying boulders to the summit of
Mount Washington, nearly 6,000 feet high, scattering others over an area
which is roughly 1,000 miles from east to west and 600 from north to south,
and in its course producing those wonderful striæ, grooves, and
furrows in the rocks photographed in the American reports, and the enormous
extent of smoothed and rounded rock surfaces that is found over this wide
area. Besides these there are two other phenomena absolutely inconsistent
with a diluvial agency. One is the enormous deposits of fine compact clay
bearing rounded and scratched stones thickly scattered through it, utterly
unlike any [[p. 627]] deposit produced by
water, which would necessarily leave the stones hundreds of miles behind
the place to which the fine mud would be carried. The other is the existence
of well-defined heaps, mounds, and ridges of gravel and boulders, forming
the terminal moraine of the ice-sheet. This is exactly similar in general
form and structure to the moraines left by the old Alpine or North British
glaciers, and if the former could have been produced by a flood so could
the latter. But the American terminal moraine runs across the country
almost irrespective of its contour, and is often as well marked on plateaus
as in valleys and on the intermediate slopes. Moreover, this moraine often
lies on the southern slope of the hills, draining towards the Mississippi
valley; and we are asked to believe that a flood vast enough to carry
gravel and rocks for hundreds of miles to such a position, left them all
stranded on a slope down which it must have been rushing with increased
velocity and without hindrance towards the Gulf of Mexico! So far as I
know, Sir Henry Howorth is absolutely alone among living writers in his
diluvial theories, and I only give this brief statement of their overwhelming
impossibilities because his book is so interesting, and his assertions
that his theory explains all the facts are so confident and so
often repeated, that they are likely to confuse the judgment of readers
who have not paid special attention to the subject.

Returning to the main
question, of the possibility of glaciers or ice-sheets moving over long
distances of generally level ground with intervening hills and valleys,
there is an important piece of evidence, the bearing of which appears
to have been overlooked by objectors. The former existence of the great
Rhone glacier carrying erratics to the slopes of the Jura from beyond
Geneva on the south-west to Soleure on the north-east, is universally
admitted. This glacier passed out of the gorge between the Dent du Midi
and the Dent de Morcles, and a little below St. Maurice enters on the
alluvial plain which extends to the lake. From this point to Geneva, a
distance of about 60 miles, may be considered a level plain, the descent
into the lake being balanced by the ascent out of it. Yet it is admitted
that the glacier did move over this distance, since erratics
which can be traced to their source on the left of the valley below Martigny
are found near that city. But the main part of the glacier curved round
to the right across the Lake of Neufchatel, and extended at least as far
as Soleure, a distance of about 90 miles. To do this it must have ascended
500 or 600 feet to the country around Fribourg, and before reaching Soleure
must have passed over a hill 300 or 400 feet higher. Yet on the flanks
of the Jura above Soleure there are erratics which have been carried on
the surface of the glacier from the east side of the valley below Martigny,
and close to Soleure itself there are [[p. 628]]
remains of a terminal sub-glacial moraine of compact boulder-clay. Sir
Charles Lyell describes this as--

"an unstratified mass of clay or mud, through which a variety of angular and rubbed
stones were scattered, and a marked proportion of the whole were polished and
scratched, and the clay rendered so compact, as if by the incumbent pressure of a great
mass of ice, that it has been found necessary to blow it up with gunpowder in making
railway cuttings through part of it. A marble rock, of the age of our Portland stone, on
which this old moraine rests has its surface polished like a looking-glass, displaying
beautiful sections of fossil shells, while occasionally, besides finer striæ, there are deep
rectilinear grooves, agreeing in direction with the course in which the extinct glacier
moved according to the theory of M. Guyot before explained."6

It is evident that,
to have produced such effects as are here described, the glacier must
have extended much beyond Soleure, and have been very thick even there.
It thus proves to demonstration that a glacier can travel for
100 miles over a generally level country, that it can pass over
hills and valleys, and that, even near its termination, it can
groove, and grind, and polish rocks, and deposit large masses of hard
boulder-clay. And all this was done by a single glacier issuing from a
comparatively narrow valley, and then spreading out over an area many
times greater than that of its whole previous course. In this case it
is clear that such a vast mass of ice, constituting a veritable ice-sheet
on a small scale, could not have derived its motion solely from the push
given to it by the parent glacier at St. Maurice. Neither could gravitation
derived from the slope of the ground have affected it, for it passed mostly
over level ground or up slopes, and its termination at Soleure is actually
nearly 200 feet higher than its starting-point at the mouth of the valley
below St. Moritz! There remains as a cause of motion only the slope of
the upper surface of the glacier, the ice slowly flowing downward, and,
by means of its tenacity and its viscosity on a large scale, dragging
its lower portion still more slowly over the uneven or upward-sloping
surface. This mode of motion will be discussed later when dealing with
the origin of lake-basins.

No doubt at this epoch
of maximum glaciation the ice-sheet extended over the whole country between
the Bernese Alps and the Jura, and the downward flow of the lateral glaciers
along the valley of the Sarine, Aare, and other rivers flowing towards
Soleure greatly assisted the general onward motion. But the fact remains,
and it cannot be too strongly insisted on, that here we have a veritable
ice-sheet moving over hill and valley, carrying on its surface quantities
of erratic blocks, rounding, striating, and polishing the rocks over which
it passed, and with the material thus crushed and ground away forming
great deposits of boulder-clay, much of which still remains, although
enormous quantities must have been carried away [[p.
629]] by the rivers to the lowlands of Europe and to the sea. The
fact is therefore demonstrated, and is implicitly admitted by the most
conservative of glacialists, that in this case an ice-sheet has
moved onward over a hilly plateau for nearly 100 miles, even when its
terminal moraine is at a higher level than its exit from the mountain
valley where it had its origin.

It will now be well
briefly to sketch the distribution of erratic blocks in Great Britain,
and the conclusions to be drawn from them as to the former existence of
an ice-sheet under which the greater part of our islands was buried.

Every mountain group
north of the Bristol Channel was a centre from which, in the earlier and
later phases of the Ice Age, glaciers radiated; but many facts prove that
during its maximum development these separate glacier systems became confluent,
and formed extensive ice-sheets which overflowed into the Atlantic Ocean
on the west, and spread far over the English lowlands on the east and
south. This is indicated partly by the great height at which glacial striæ
are found, reaching to 2,500 feet in the Lake District and in Ireland,
somewhat higher in North Wales, and in Scotland to nearly 3,500 feet;
but also by the extraordinary distribution of erratic blocks, many of
which can be traced to localities whence they could only have been brought
across the sea. The direction of the glacial striæ and of the smoothed
side of ice-worn rocks also indicate that the shallow seas were all filled
up by ice. The Outer Hebrides, for example, are all ice-ground from the
south-east and east, showing that the deep channel of the Minch was filled
up, and that the Scotch ice-sheet flowed completely over the islands.
On all sides of Ireland, except the southern coast, the ice flowed outward,
but on the north-east the flow was diverted southward, and on the extreme
north, westward, by the pressure of the overflowing ice-sheet of Scotland
which here encountered it. In like manner, the ice-marks on the east coast
of Ireland and the west coast of Wales are diverted southward by the mutual
pressure of their ice-sheets, which, together with that of the west of
Scotland, filled up St. George's Channel. That such was the case is further
proved by the fact that the Isle of Man is ice-ground in a general direction
from north to south, and to the summit of its loftiest mountains which
rise to a height of over 2,000 feet. This could only have been done by
an ice-sheet flowing over it, and this view is further supported by some
most remarkable facts in the dispersal of local erratics. These
are always found to the south of the places where they occur in situ,
never to the north; and, what is still more noteworthy, they are often
found far above the native rock. Thus, boulders of the peculiar Foxdale
granite are found about 1,400 feet higher than the highest point where
there is an out-crop of this rock.

[[p. 630]] The Scotch ice-sheet flowed outwards on all sides, but
on the east it was met by the southward extension of the great Scandinavian
ice-sheet. On the extreme north the meeting of these two ice-sheets resulted
in a flow to the north-west which glaciated the Orkney Islands, while
the Shetlands, much farther north, received the full impact of the Scandinavian
ice alone, and are therefore glaciated from the north-east. The dividing-line
of the Scotch and Scandinavian ice-sheets was in the North Sea, not far
from the east coast of Scotland; but farther south, at Flamborough Head
and Holderness, the latter impinged on our coast, bringing with it enormous
quantities of Scandinavian rocks. Many years ago Professor Sedgwick described
the cliffs of boulder-clay at Holderness as containing "an incredible
number of smooth round blocks of granite, gneiss, greenstone, mica slate,
&c., &c., resembling none of the rocks of England, but resembling
specimens derived from various parts of the great Scandinavian chain."
These are mixed, however, with a number of British rocks from the north
and west, indicating the meeting ground of the two conflicting ice-sheets.
Similar blocks occur all along the coast as far as the cliffs of Cromer
in Norfolk. Across the peninsula of Flamborough about two miles west of
the lighthouse there is a moraine ridge containing a few Scandinavian
boulders, but mainly composed of British rocks. These latter consist of
numerous carboniferous rocks from the north and north-west, together with
many of Shap granite--a peculiar rock found only on Shap Fell in the eastern
side of the Lake District, together with a few of Galloway granite. These
facts, it will be seen, add further confirmation to the theory of great
confluent ice-sheets indicated by the ice-markings upon the various groups
of mountains, while it is hopelessly impossible to explain them on any
theory of local glaciers, even with the aid of submergence and of floating
ice.

The study of our British
erratics has been assiduously pursued for many years past by a committee
of the British Association; and by means of a map showing the chief facts
collected up to this date, kindly furnished me by Mr. Percy F. Kendal,
secretary of the committee, I am able to give a brief sketch of the more
important of the phenomena, and their bearing on the extent and motion
of the British ice-sheet. The general reader may be informed that great
numbers of rocks are so local and so characteristic, often being confined
to a very limited district or to a single mountain, that the origin of
a considerable portion of the erratics can be ascertained with the greatest
certainty.

Taking first the Shap
granite, which has already been mentioned as occurring at Flamborough
Head, we find that it has been carried northwards as far as the Solway
Frith, and eastward to the Eden valley in great quantity and over a wide
area. Thence can be [[p. 631]] traced a line
of boulders of this rock over the high plateau of Stainmoor into the valley
of the Tees, and onward round the coast by Scarborough to Holderness,
while a branch descends southward along the valley of the Ouse to York.
Coming back to its source on Shap Fell, a train of boulders of the same
rock has been traced south-ward in a curving line, passing the east side
of Morecambe Bay near Lancaster, and thence sparingly south-eastward to
near Whalley. Along the same line are found boulders of peculiar granites
from Eskdale and Buttermere, marking the line of junction of the northern
ice-sheet with that which filled up the Irish Sea and pressed inward between
the glaciers of Cumberland and North Wales. This is indicated by the fact
that south of this line are scattered immense quantities of erratics,
both from the south-west of Scotland and the Lake District, spreading
over the whole of the low country as far as Bridgnorth and Wolverhampton,
and eastward to the Derbyshire highlands. These same erratics are found
round the north coasts of Wales and part of Anglesea, showing how the
iceflows divided on either side of the mountain mass of North Wales.

The centre of the great
glacier sheet of North Wales appears to have been over the Arenig Mountains,
whence erratics of a peculiar volcanic rock have been traced to the north
and east, mingling with the last-described group; while a distinct train
of these Welsh erratics stretches south-eastward to the country west of
Birmingham.

In the Isle of Man are
found many erratics from Galloway and a few from the Lake District. But
the most remarkable are those of a very peculiar rock found only on Ailsa
Craig, a small island in the Frith of Clyde, and a single boulder of a
peculiar pitchstone found only in the Isle of Arran. The Ailsa Craig rock
has also been found at Moel Tryfaen on the west side of Snowdon, and more
recently at Killiney, co. Dublin, on the seashore.7

The case of the boulders
in the Isle of Man, which have been carried nearly 800 feet above their
source, has already been mentioned, but there are many other examples
of this phenomenon in our islands; and as they are of great importance
in regard to the general theory of glacial motion a few of them may be
noted here. So early as 1818 Mr. Weaver described a granite block on the
top of Cronebane, a slate hill in Ireland, and several hundred feet higher
than any place where similar granite was to be found in situ;
and he also noticed several deposits of limestone gravel in places from
300 to 400 feet higher than the beds of limestone rock which are from
two to ten miles off. Debris of red sandstone is also found much higher
than the parent rock. Boulders of Shap granite, Mr. Kendal tells us, have
passed over Stainmoor by tens of thousands, and in doing so have been
carried about 200 feet above their source; and the curious Permian rock,
"Brockram," has been carried in the [[p. 632]]
same direction no less than 1,000 feet higher than its highest point of
origin.8 In Scandinavia there are still more striking examples,
erratic blocks having been found at an elevation of 4,500 feet which could
not possibly have come from any place higher than 1,800 feet.9
We thus find clear and absolute demonstration of glacier ice moving up-hill
and dragging with it rocks from lower levels to elevations varying from
200 to 2,700 feet above their origin. In Switzerland we have proof of
the same general fact in the terminal moraine of the northern branch of
the Rhone glacier being about 200 feet higher than the Lake of Geneva,
with very much higher intervening ground. As it is universally admitted
that the glacier of the Rhone did extend to beyond Soleure all the a
priori objections to the various cases of rocks carried much higher
than their origin, in America, the British Isles, and Scandinavia, fall
to the ground. We must either deny the existence of the ice-sheet in the
great Swiss valley, and find some other means of accounting for the travelled
blocks on the Jura between Geneva and Soleure, or admit that the lower
strata of a great glacier can travel up-hill and over hill and
valley, and that the ice-sheets of the British Isles, of Scandinavia,
and of North America merely exhibit the very same characteristics as those
of Switzerland, but sometimes on a larger scale. We may not be yet able
to explain fully how it thus moves, or what slope of the upper surface
is required in order that the bottom of the ice may move up a given ascent,
but the fact of such motion cannot any longer be denied.

The facts thus established
render it more easy for us to accept one of the latest conclusions of
British glacialists. A great submergence of a large portion of the British
Isles during the glacial period or in the interval between successive
phases of the glacial period, has long been accepted by geologists, and
maps have been often published showing the small group of islands to which
our country was then reduced, the supposed subsidence being about 1,400
feet. The evidence for this is the occurrence, at a few spots, of glacial
gravels containing marine shells in tolerable abundance, the most celebrated
being at Moel Tryfaen, on the west side of Snowdon, at a height of more
than 1,300 feet. Shell-bearing drifts have also been found near Macclesfield
at a height of over 1,100 feet, and to the east of Manchester at between
500 and 600 feet elevation. Others have since been found on Gloppa, a
hill near Oswestry. The fact that the shell-bearing gravels of Moel Tryfaen
are nearly 40 feet thick shows that, if they are due to submergence, the
land must have remained stationary at that level for a considerable period
of time, and there would probably be other stationary periods at lower
levels. Yet nowhere in the valleys or on the hill slopes of Wales, [[p.
633]] or the Lake District, or in the English lowlands are there
any of the old beaches or sea cliffs, or marine deposits of any kind,
that must have been formed during such a subsidence and which can hardly
have been everywhere cleared away by subsequent glaciation. Another difficulty
is that the shells of these drifts are such as could not have lived together
on one spot, some being northern species others southern, some frequenting
sandy others muddy bottoms, some which live only below tidal water while
others are shore species. And, lastly, they are very fragmentary, only
a small percentage of entire shells being found.

In consequence of these
various difficulties it was suggested by the late Mr. Belt that the great
Irish Sea ice-sheet had carried up a portion of the sea-bottom embedded
in its substance, perhaps containing deposits of shells of various periods
and thus explaining the intermixture of species as well as their fragmentary
condition. The fact that boulders and pebbles from Scotland, Ailsa Craig,
and Cumberland have been found in the Moel Tryfaen beds almost amounts
to a proof that they were so uplifted; and a recent search has shown that
in the other localities where marine shells have been found in drift at
great elevations similar foreign rocks occur, rendering it almost certain
that the same ice-sheets which have distributed foreign erratics so widely
over our country, and which in doing so must have passed over
the sea-bottom, have in a few cases carried with them a portion of that
sea-bottom, and deposited it with the erratics in the places where both
are now found. A full discussion of this point, with replies to various
objections, by Mr. P. F. Kendal, will be found in the volume already quoted;
and he has recently adduced a fresh argument against "the great submergence"
in the fact that, if it ever occurred, our lowlands must for a long time
have formed the bottom of a sea 200 fathoms deep, yet not a single shell
characteristic of that depth has yet been discovered in the drift.10
The cumulative evidence against the submergence is now almost, if not
quite, conclusive.

In the brief outline
now given of the facts of glacial geology bearing upon the former existence,
the thickness, extent, and motion of ice-sheets, it has only been possible
to treat the subject very broadly, omitting all those details and minor
difficulties which cannot be discussed within the limits of a popular
article. My object has been to explain the nature and amount of the converging
evidence demonstrating the existence of enormous ice-sheets in the northern
hemisphere, to serve as a basis for the discussion of the glacial origin
of lake-basins, which will form the subject of another article.

Notes Appearing in the Original Work

1. The works referred to are:--Do Glaciers Excavate? by Prof. T. G. Bonney, F.R.S. (The Geographical
Journal, vol. i., No. 6); The Glacial Nightmare and the Flood, by Sir H. H. Howorth, M.P., F.R.S.; Fragments
of Earth Lore, by Prof. James Geikie, F.R.S.; Man and the Glacial Period, by Prof. G. F. Wright, F.G.S.A.; La
Période Glaciaire, by A. Falsan; and the Glacialists' Magazine, edited by Percy F. Kendall, F.G.S.; from which
works, and from those of Lyell, Ramsay, Geikie, and the American geologists, most of the facts referred to in
the present article are derived. [[on p. 616]]

2. Seventh Annual Report of the United States Geological Survey, p. 179. Arrangements have now been made
for the preservation of these remarkable examples of ice-work. [[on p. 621]]

3. A map showing the lines of dispersal of these erratics is given in Lyell's Antiquity of Man, p. 344, and is
reproduced in my Island Life, p. 111. [[on p. 623]]

4. These figures are almost certainly incorrect, as the upper surface of the glacier must have had a considerable
downward slope to produce motion. The recent work of M. Falsan, La Période Glaciaire, gives the thickness as
about 3,800 feet at the head of the lake and 3,250 feet at Geneva. [[on p. 623]]

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